Detection of arcs in power cables

Abstract: A process detects arc discharges in a power cable (12) connecting a power supply board (10) and an equipment rack (11), the power supply board including a circuit breaker (13) operable at a preset current level, and said arc discharge occurring at a fraction of said current level. After the arc discharge is detected (by 15), a signal representing the presence of the arc discharge is supplied to a power supply control (16), and a control signal is provided by the power supply control to cause the circuit breaker (13) to open interrupting supply of power to the affected power cable.

Description: This invention relates to methods of detecting arc discharges in power cables connecting a power supply board and at least one equipment rack. Central telephone exchange offices utilize DC power to run various telephone communication equipment. DC power is brought into the office to a main switch (or power supply) board from where it is directed to various equipment by power cables. The separation between the main switch board and the power cables is provided typically by DC circuit breakers intended to shut down the equipment in case of short circuits. Very often an arcing arises between conductors in the cable which if undetected leads to a fire first hidden inside the cable cover and subsequently to an observable fire consuming the cable. Arc discharges within a cable may arise due to a number of factors one of which could be a fault in the insulation on separate conductors in a multiconductor cable. Such a fault could be a result of a manufacturing flaw or damage to the conductor within the cable. Such damage may occur in the course of transportation, installation or use of the cable. Stepping on, kicking or some other damaging act to the cable either during the installation or use in the office may be a contributory cause of the injury. Arc discharges which may begin as an occasional sparking between adjoining conductors within the cable, if undetected, may lead to a fire within the cable. Arc discharges occur at relatively low current levels of from 0.5 to 10 amperes. However, a typical central office circuit breaker operates at 40-50 ampere level. Thus, the circuit breakers do not recognize such arc discharges permitting the arc discharges to continue with resultant overheating of the cable leading to a fire of the cable and consequently in the central office. It is desirable to timely detect occurrence of arc discharges in DC power cable and to shut down the equipment prior to occurrence of such arc discharges which could lead to cable fires. According to this invention there is provided a method as claimed in claim 1. This invention embodies a process for detecting arc discharges in power cables connecting a power supply board and at least one equipment rack, the power supply board including circuit breakers operable at a certain current level, said arc discharge occurring at a fraction of said current level. After the arc discharge is detected, a signal representing the presence of the arc discharge is supplied to a power supply control, and a control signal is provided by the power supply control to cause said at least one circuit breaker to open interrupting supply of power to an affected power cable. Brief Description of the Drawings FIG. 1 is a schematic representation of an arc discharge detection system utilizing detection of a negative resistance characteristic of the arc discharge; FIG. 2 is a schematic representation of the arc discharge detection system of FIG. 1 for use with a plurality of power cables; FIG. 3 is a schematic representation of a variant of an arc discharge detection system of FIG. 1; and FIG. 4 is a schematic representation of an exemplary circuit utilized for the power supply control. Detailed Description DC arcs have characteristics by which they could be recognized and in response to this recognition circuit breakers could be operated which would cause shut off supply of power to the affected power cable. Electronic circuits that recognize these characteristics can be used to affect the circuit breakers and to prevent fires. A "negative resistance" is an arc discharge characteristic that may be used to detect an arcing power cable. In order to detect the negative resistance, a tuned circuit is placed across the power cable. The tuned circuit includes a capacitor (C), a resistance (R) and an inductance (L). The tuned circuit has a positive resistance. An active DC arc exhibits a negative resistance, that is, as the voltage across an arc increases, the current drops. An arc violates Ohm's Law. Simply stated Ohm's Law says: the electric current passing through an element is directly proportional to the voltage across it and inversely proportional to its resistance. However, an arc between two electrodes (or two wires) does not behave this way. As the current passing through the arc increases, the voltage between the two electrodes decreases so that the current and the voltage are inversely, not directly, proportional. Thus the arc has a "negative resistance". If the negative resistance of the arc is added to the positive resistance of a tuned circuit, the circuit will oscillate or generate an excessive noise. This signal can be amplified and detected, and the detected signal can be used to shut-down the supply of power to the power cables. An exemplary tuned circuit may have a capacitance C within a range from 0.01 microfarad to 100 microfarad, a resistance R within a range of from 1 ohm to 25 ohm, and an inductance L within a range of from 50 microhenries to 100 millihenries. Arcing in a typical power line provided with the tuned circuit would lead to oscillations within a range of from 750 Hz to 10 KHz. Shown in FIG. 1 is an arrangement with a power supply 10, an equipment rack, 11, and a power cable, 12. The detection system includes a tuned circuit, 31, a band pass amplifier, 32, and a detector, 33. The tuned circuit includes a capacitor C, 36, a resistance R, 37, and an inductance L, 38, of a loop coupling the tuned circuit to the power cable. Arcing in the cable, at 39, would cause a negative resistance (R 1 ) which when added to the positive resistance (R) of the tuned circuit, would result in a signal which when supplied via a band pass amplifier 32 and detector 33 to a power supply control, 16, shall result in a control signal to open circuit breaker 13 and to interrupt supply of power to the power cable. In FIG. 2 is shown an arrangement which includes a plurality of detection systems shown in FIG. 1 and which is designed to overview each one of a plurality of power cables 12A and 12B, separately, provide a signal representative of the presence of an arc in the affected power cable to power supply control 16. The latter in turn provides a control signal to power supply board 10 causing an affected one of the separate circuit breakers 13A and 13B to disconnect supply of power only to the affected power cable. In this manner the remaining equipment rack may continue to remain operational. The implementation of FIGs. 1 and 2 is useful when there are connection points to which tuned circuit 31 can be coupled. In this implementation the tuned circuit is placed directly across the D.C. power line. This may conveniently be done at one of the connectors to the electronic rack or at a cable junction. However, the preferred location is at some distance (3-20 feet) from the power supply. This minimizes loading or shunting of the arc by the internal impedance of the power supply. If a choke (not shown) is placed in one or both of the leads of the power cable, the sensitivity to arc detection will be improved. Under this circumstance the location of the tuned circuit in the D.C. cable becomes unimportant. The gain of the amplifier should be as high as possible, but not so high that a false shut down of the equipment is likely. This would be the case if the cable was located in a high RF environment. A typical gain for the band pass amplifier would be 20-50 db. FIG. 3 is an implementation of a negative resistance detection system that is most useful when one has limited access to a cable connection or when one cannot easily "cut into" the cable. Tuned circuit 31, band pass amplifier 32, detector 33 and power supply control 16 are similar to those shown in FIGs. 1 and 2. However, here one or both leads of cable 12 pass through or encircle a transformer core, 51, as a primary winding, 52. A typical core might be a ferrite or powdered iron toroid. A secondary winding, 53, of the toroid, connected to the tuned circuit, typically gives a 10:1 step-up ratio. Again, a typical gain for the band pass amplifier would be 20-50 db. An exemplary band pass amplifier for the detection system disclosed in connection with FIGs. 1, 2 and 3 may include two MAR-8 Mini-Circuits amplifiers in series, each of which is preceded by a PLP-5 Mini-Circuits low pass filter. The output of the amplifiers is fed to an HP 423 B crystal detector. Coupling capacitors (0.001 pfd) are placed between the stages to limit the low frequency response to a few hundred Hz. Another implementation replaces the band pass amplifier with a PALOMAR VLF Converter obtainable from PALOMAR Engineers, Escondido, California. Power supply control 16 shown in FIG. 1, 2 and 3 utilizes a relay system, 21, which in response to a signal from a detector diode (FIGs. 1, 2 and 3) causes circuit breaker 13 or 13A or 13B to open so as to interrupt supply of power from the power supply to a corresponding equipment rack 11. One example of such a circuit for the power supply control is shown in FIG. 4. The signal from the diode detector, which is caused by the arc, may be random, continuous or discontinuous. A first transistor, 22, amplifies the signal. A second transistor, 23, adds some time delay before the third transistor causes relay, 25, to close affecting circuit breaker to break the circuit. The delay ensures that the relay closes only on persistent signals from the arc and not on transient signals from other sources. The circuit of FIG. 4 is commonly called a carrier operated relay (COR). Other names for a COR are squelch relay, squelch-operated relay, squelch gate or signal-operated relay. COR's, frequently used in FM mobile radio transceivers and commercial units, are available as replacement circuit boards, for example, from MOTOROLA, Phoenix, Arizona. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described.

Claims: A method of detecting arc discharges in a power cable connecting a power supply board and at least one equipment rack, said power supply board including a circuit breaker operating at a preset current level, and said arc discharge occurring at a fraction of said current level, the method including supplying a signal representing the presence of the arc discharge to a power supply control, and providing a control signal from said power supply control sufficient to cause said circuit breaker to open thereby interrupting supply of power from the power supply board to said power cable, wherein said arc discharge detection is provided by coupling a tuned circuit between said power cable and ground, said tuned circuit including a capacitor (C), a resistance (R) and an inductance connected in series and having a positive resistance, and said arc discharge has a negative resistance which counteracts the positive resistance resulting in oscillations or noise of the tuned circuit supplying said signal to the power supply control. A method as claimed in claim 1 wherein said arc discharge detection is provided for each of a plurality of power cables.

Inventors: FLEMING DEBRA ANNE, PETERSON GEORGE EARL, FLEMING, DEBRA ANNE, PETERSON, GEORGE EARL
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: G01R31
IPC codes: G01R31, G01R29, H02H1